Susceptors are employed in the preparation of food products in microwave ovens to convert some of the microwave energy to heat in order to assist in cooking the food by conduction, convection and/or radiant heating as well as microwave radiation. Susceptors may comprise the cooking surfaces of kitchen utensils, the bottom of packaged food products, such as unpopped popcorn, and a food wrap for a food product, such as meat or bread, which when cooked desirably should have a browned or crisped exterior surface.
Susceptors frequently comprise or are included in the packaging for food products as a convenience to the consumer, so that the consumer can simply pop the product into a microwave oven without any significant preparation. As a further convenience, such packaging is customarily disposable. Thus, there is a particular need for susceptors that are economical.
However, since susceptors will be brought into contact with foods intended for human consumption, it is necessary to encapsulate the microwave interactive material within films or the like that are approved for contact with food, thus resulting in a multi-layer susceptor product. Customarily, the susceptor product comprises a base sheet such as paper, cardboard, box board or the like, a thin film or foil of microwave interactive material, such as aluminum and other selected metals and alloys, and a heat resistant barrier film overlying the metal film or foil.
The multi-layer sheet may then be die cut or pressed into boxes or trays and/or decorated with printing to form a package into which food may be inserted by a food processor. Alternatively, the multi-layer sheet may comprise a flexible laminate which can be formed around a food product as a wrapping material at the food processor's plant.
The barrier film is typically a polyester because of its heat resistant properties and low cost. However, the barrier film may also be polyimide, cellulose, polyethylene nitrile and other heat resistant films. Its purpose is to provide a functional barrier between the food product and the metal, and sometimes also to serve as a sealable layer to facilitate formation of a package.
The microwave interactive susceptor material is typically a metal or metal alloy or derivative, in single or multi-layer formations, but also may be ceramic or carbon. Any element or compound that absorbs the electromagnetic microwave energy and converts it to radiant heat is suitable. The metals are usually applied by using evaporative or sputtering deposition methods. Flakes of metal are sometimes applied in a rotary printing process. Ceramics and carbon may also be applied in a rotary printing process.
Heretofore, it has been fairly common practice to deposit a film or layer of the microwave interactive material onto the barrier film, e.g., a web of polyester film, and to then laminate the metallized film onto a web of supporting substrate material, usually board, paper or cellulose.